1. Field of the Invention
The present invention relates generally to integrated optics, and, more particularly, to a system for tuning electrooptically activated integrated optical switch elements so as to optimize the switching performance of the device.
2. Background of the Invention
Optical switch elements, such as directional couplers, X-branch, Y-branch, and balanced bridge optical switches are well known. Typically, an integrated optical switch element is formed of an electro-optic material, such as lithium niobate, which has at least one wave guide channel defined within an interaction region of the electro-optic material for guiding light signals through the interaction region. In order for the wave guide channel(s) to be configured as an optical switch element, there must be at least a first input port into which a light signal is selectively propagated and a second input port. At least a pair of electrodes are used to selectively produce an electric field across the interaction region that electro-optically actuates a desired switch state of the optical switch element.
The most common configurations for optical switch elements are a 2.times.1 element, e.g., a "Y-branch" switch or a balanced bridge switch element, and a 2.times.2 element, e.g., a directional coupler or "X-branch" optical switch element. In each case, there are two "input" ports which can receive a light signal and then selectively switch the light signal in the interaction region of the switch element to one of the "output" ports. Because optical switches are generally bi-directional in that light can propagate through the switch in either direction, the determination of whether a port of an optical switch element is an input port or an output port is entirely definitional. A port defined as an input port in one context could be defined as an output port in another context if, for example, the light signal were to enter the switch element on the other side of the interaction region.
The wave guide channels defined in the electro-optic material transmit light signals from the input ports through the interaction region where the light signals may be routed to or away from any of the output ports, depending upon the desired switch state of the optical switch element as determined by the electrical field created by a voltage applied to the electrodes at the interaction region of the device. For example, a light signal entering an input port for a first wave guide channel can be transferred within a 2.times.2 directional coupler so as to exit at an output port for a second wave guide channel, in which case the switch element is said to be in a "cross state". Alternatively, the light signal entering the input port for the first wave guide channel can be passed through the 2.times.2 directional coupler so as to remain in the first wave guide channel and exit at an output port for the first wave guide, in which case the switch element is said to be in the "bar" state.
Ideally, an electro-optically activated optical switch element is designed so as to electro-optically switch the light signals between the desired switch states in response to an electric field that is applied across the interaction region where the wave guide channels intersect or are very close to one another. In this region, the wave guide channels are constructed so as not to restrict the light signal to staying within a particular wave guide channel. Consequently, a light signal passing through the interaction region is free to stay within one wave guide channel, cross over to another wave guide channel, or do both, depending upon the optical transmission properties of the wave guide channels within the interaction region. When an electric field is applied across the interaction region of an electro-optic material, the electric field changes the optical transmission properties of the wave guide channels. As a result, the way in which the light signal passes through the interaction region can also be changed. Examples of how existing electrically-activated optical switch elements are configured and operated are shown in U.S. Pat. Nos. 5,050,947 and 5,255,334.
While it would be desirable if optical switch elements operated in a completely digital manner; being either completely on or completely off, the control of the light signal as it passes through the optical switch element is not that simple. In practice, an electro-optically activated optical switch element behaves more like a leaky valve, with most of the light signal being transferred through the desired wave guide channel, but with some of the light signal leaking into the other wave guide channel(s). As long as the relative difference between the optical power propagated to the outputs of each of the wave guide channels is large enough, it is still possible to use the optical switch element in a digital manner. When this relative difference between the optical power of each of the outputs is expressed as a ratio, it is often referred to as the switching extinction ratio of the optical switch element.
Although it would be desirable if only the voltages applied to the electrodes affected the relative difference between the optical outputs, there actually are many factors which can affect the switching extinction ratio of an integrated optical switch element. Some of these factors can be controlled in the design of the optical switch element, such as the physical characteristics of the optical switch element; including wave guide channel length and separations; the properties of the electro-optic material; and the manner in which the wave guide channels are created in the electro-optic material. There are other factors, however, which are not capable of being controlled in the design of the optical switch element, such as the environmental conditions under which the optical switch element is operated.
With this background in mind, some of the various attempts which have been made to control the manner in which electro-optically activated integrated optical switches are used will be summarized.
U.S. Pat. No. 5,283,842 describes a method for physically trimming or altering the wave guide channels in order to affect the operation of the optical switch element. U.S. Pat. Nos. 4,769,534 and 5,023,445 describe methods for providing a feedback signal from a photodetector at an output port of the optical switch element. The feedback signal is used to mask a laser applied to an input port of the optical switch element in situations where the optical switch element is used as part of an optical time domain reflectometer (OTDR). In each of these references, however, no consideration is given to control of the voltages applied to the electrodes used to electro-optically alter the desired switch state of the optical switch element.
Control of the voltages applied to the electrodes of an integrated optical switch has generally been limited to techniques used to control or minimize voltage induced drift, such as described in U.S. Pat. Nos. 5,020,872 and 5,218,468. In each of these patents, the input voltages to a pair of electrodes used in connection with an optical switch element are altered such that there is a zero average of the input voltages over time. By using an average potential difference between the input voltages that is substantially zero, these patents attempt to minimize any drift in the desired operating point voltages of the optical switch element that might be induced by applying a non-zero average voltage to the electrodes over a long period of time. In each of the patents, the zero averaging of the control voltages is accomplished without the use of any type of feedback from the optical switch element.
The use of an input tap off from an optical device as part of a mechanism to control an optical switch element is described in U.S. Pat. No. 5,218,198. In the system described in this patent, there is a tap off of a fraction of an input signal that is sent to an electronic circuit so as to monitor the input signal in order to decide if switching of the optical switch element is necessary. While the tap off signal, and the associated electronic circuitry, are used to control the desired switch state of the optical switch element, the control is accomplished in response to the information contained in the input signal and no attempt is made to control the electrode voltages to compensate for any other factors that might affect the operation of the optical switch element.
In U.S. Pat. No. 5,259,044, the scattering of a light signal is used by an optical device that is not an optical switch element in an attempt to control a DC bias voltage applied to control electrodes on the device. The optical device is a one-input/one-output Mach-Zehnder optical modulator which operates such that a light signal is either allowed to pass from the input port through the modulator to the output port, or is attenuated. In this patent, a photodetector is located on the output side of the interaction region attached to a side face or to an output port of the Mach-Zehnder optical device. The purpose of this photodetector is to detect a scattering of light by the device at the output port. This scattered light is monitored for the purpose of controlling a DC bias voltage applied between a first and second control electrode. Because a Mach-Zehnder optical modulator does not perform any "switching" functions, and because the scattered light signal never passes back through the interaction region of the device, the manner in which the scattered light signal is detected and monitored as taught by this patent is of little use in controlling the operation of an integrated optical switch element.
While there is a growing interest in the use of electro-optically activated integrated optical switch elements for optical devices and systems, there has been little effort directed to the manner in which such optical switch elements can be controlled to optimize their performance. Consequently, there is a need for a system for tuning an integrated optical switch element that can be used to optimize the performance of the integrated optical switch elements.